Ignition Coil Drive Circuit

[Bob Huish]

I started building and using CD ign. systems in the late 50's. The 
two main advantages were extended point life, (no longer and issue) 
and a much hotter spark to ignite the fuel at 8000 rpms. A side 
benefit (for me that is) was the ability to increase the plug gap to 
.075 which not only increased HP, but gave large increases in fuel 
mileage.

The CD systems normally dumped 450 volts across the coil to ~ 3000 
rpms then the inverter output would taper off to 225 volts @ 8000 
rpm's (8 cyl). Stock coils had a tendency to "smoke" and the early 
xisor coils as well. (many were rated @ between 250 and 400:1 turns 
ratios)

Post 70's coils work well and improvements in transformers and their 
drive transistors made the CD voltages more consistent 
(400-300volts), across the 0-8000 rpm range. 

Presently most if not all, major ignition component mfgs, Accel, 
Edlebrock, Jacobs, Mallory, MSD make CD type systems. Many touting 
their ability to tailor the output voltage to engine load.  The 
desirability of this feature is open for discussion.

About 10 years ago I tested three different types of systems, an "old 
Mallory Mag Spark, MSD CD, and a dual coil system. Starting, mid 
range and short burst to 7000 rpm's, they all worked well, but
according to the "seat of the pants" meter (and sort of stop watch  
verified) the dual coil performed best, followed by the Mag Spark and 
lastly the MSD. Note: All out performed the stock xistor system.

It should be noted that any improvement to spark generation requires 
top quality wires and special attention to their routing, mounting 
insulators and connections at both ends. A top quality cap and rotor 
assembly is needed too.
Bob Cuda-65 - Angels Camp, Calif.
cudabob at workmail.com
http://www.goldrush.com/~rhuish/